Catastrophic breakdown injury exacts a major toll on Thoroughbred racing as a contemporary sporting event and represents a significant mortality risk for individual horses. Predisposition to fracture dislocation of the bones forming the metacarpophalangeal articulation have led to studies of why bones break, but little progress has been made in accurate detection of stress fracture, trabeculae irregularity, or soft tissue injury that may set the stage for catastrophic failure during intense exercise. Clinical lameness and swelling often precede more serious injury, and veterinary screening prior to racing is mandated in most jurisdictions. What follows a failed vet check varies depending on trainer and attending veterinarian. Routine radiographs and ultrasound are used frequently, but often fail to elucidate an exact cause for clinical symptoms around the fetlock. More advanced imaging tools are available, including nuclear scintigraphy, standing MRI, and immunoassay of circulating markers of excessive bone turnover. These modalities are used intermittently as sophisticated screening tools, but a comprehensive study of their combined role in identifying high risk lesions has not been done. This proposal will use nuclear bone scan, standing MRI, and serum bone markers to provide a combinatorial approach to assess horses failing a pre-race vet check. We hypothesize that these imaging modalities will provide a sensitive screening tool to minimize racetrack breakdown. The value of individual imaging modalities such as bone scan are already apparent in detection of stress fracture in the proximal long-bones of the skeleton. Much less is known about MRI in racehorse stress injury, particularly in the lower limb, and correlative studies of multiple imaging techniques have not been reported. We propose to address this by enrolling race horses with suspected lower limb injury for a combined multi-imaging and serum biomarker panel screening.